1. Field of the Invention
The present invention relates to an electroluminescent lamp of a three-electrode structure which can be effectively used for example as a back light for a liquid crystal display, and more specifically, to a structure of a lead conductor for a so-called "third electrode" of the three-electrode type electroluminescent lamp.
2. Description of Related Art
In the prior art, an electroluminescent lamp having a three-electrode structure effective in preventing mechanical vibration or noise and in giving an electrostatic shield has been known. Referring to FIGS. 1 and 2, there are diagrammatically shown a plan view and a sectional view of one three-electrode type electroluminescent lamp which is known to the inventors but has not yet been known to public.
The electroluminescent lamp is generally indicated by Reference Numeral 10, and includes an electroluminescent panel 12 consisting of a phosphor dispersed organic sheet-like member 14 sandwiched between a transparent sheet-like electrode 16 and a backing metal electrode 18. The transparent electrode 16 constitutes a front side electrode through which a light emitted from the phosphor is outputted. The backing electrode 18 constitutes a rear side electrode having a function of reflecting the light emitted from the phosphor toward the transparent electrode. A base end 20A of a first or front side lead conductor 20 is laid over an edge region of the transparent electrode 16 and electrically connected to the edge region of the transparent electrode 16. The front side lead conductor 20 extends outwardly from the edge region of the transparent electrode 16. A base end 22A of a second or rear side lead conductor 22 is laid over and electrically connected to an edge region of the backing electrode 18 at the same side as that of the edge region of the transparent electrode 16 connected to the base end 20A of the lead conductor 20. A connection position between the base end 22A of the rear side lead conductor 22 and the backing electrode 18 is shifted apart from a connection position between the transparent electrode 16 and the lead conductor 20 in a direction along the edge of the backing electrode 18. The rear side lead conductor 22 extends outwardly from the edge region of the backing electrode 18 in parallel to the front side lead conductor 20.
The above mentioned electroluminescent panel 12 is enclosed and sealed within an enclosure which is formed of a transparent insulative film 24 and a rear side insulative film 26 bonded at their periphery to each other.
On a rear surface of the electroluminescent lamp, namely, on the rear side insulative film 26, a sheet-like "third electrode" assembly 28 is bonded for example by a both-surface adhesive tape (not shown). This third electrode assembly 28 includes a metal sheet 30 sandwiched between and laminated with a pair of protection films 32 and 34. For electrical connection, a base end of 36A of a third lead conductor 36 is laid over and electrically connected to an edge region of the metal sheet 30 at the same side as that of the edge region of the transparent electrode 16 connected to the base end 20A of the lead conductor 20 and at a position corresponding to a connection position between the transparent electrode 16 and the front side lead conductor 20. The third lead conductor 36 extends outwardly from the edge region of the metal sheet 30 so as to overlap the front side lead conductor 20 as shown in the plan view of in FIG. 1 when the third electrode assembly 28 is bonded to the electroluminescent panel 12.
With the above mentioned arrangement, the third lead conductor 36 extending from the third electrode assembly 28 is electrically connected to the front side lead conductor 20 extending from the transparent electrode 16. This connection puts the third electrode at the same potential as that of the transparent electrode 16. As a result, generation of mechanical vibration or noise is effectively suppressed, and an electrostatic shield is attained.
However, as seen from the plan view of FIG. 1, the third electrode assembly 28 is bonded to the electroluminescent lamp 10 in such a manner that the third lead conductor 36 overlaps the base end 20A of the front side lead conductor 20, namely a stacked connection portion between the transparent electrode 16 and the front side lead conductor 20. Therefore, a thickness of the stacked connection portion between the transparent electrode 16 and the front side lead conductor 20 has been apparently increased by a thickness of the third lead conductor. As a result, the overall thickness of the electroluminescent lamp apparatus has been correspondingly increased.
In addition, the third lead conductor 36 is fabricated independently of the metal sheet 30 of the third electrode assembly 28, and mechanically fixed and electrically connected to the metal sheet 30. If the third lead electrode 36 were previously shaped to avoid or detour the stacked connection portion 20A between the transparent electrode 16 and the front side lead conductor 20, the third lead electrode 36 thus shaped would become difficult to handle, and the cost for manufacturing the third lead electrode 36 would be increased. Therefore, even if this method were effective in decreasing the overall thickness of the electroluminescent lamp apparatus, it is not so practical.
Another example of the three-electrode type electroluminescent lamps known to the inventors has been provided with a printed-wiring board on which a connector is mounted or connection pads are formed for an external connection. One typical example of printed-wiring board has first, second and third printed wirings extending from a connector mounted region toward an edge of the printed-wiring board. A front side lead conductor, which is electrically connected at one end to a transparent electrode of electroluminescent lamp, is soldered at its other end to the first printed wiring of the printed-wiring board, and a rear side lead conductor, which is electrically connected at one end to a backing electrode of electroluminescent lamp, is also soldered at its other end to the second printed wiring of the printed-wiring board. Furthermore, a third lead conductor, which is electrically connected at one end to a so called third electrode, is also soldered at its other end to the third printed wiring.
As mentioned above, the front side lead conductor, the rear side lead conductor and the third lead conductor are soldered to the corresponding printed wirings on the printed-wiring board. Therefore, the front side, rear side and third lead conductors must have been made of a solderable metal. Therefore, if it is considered to constitute the third lead conductor with an extension of the third electrode, the third electrode would have to be formed of metal.
Here, consider respective functions of the front side lead conductor for the transparent electrode, the rear side lead conductor for the backing electrode and and the third lead conductor for the third electrode. The front side and rear side lead conductors are terminals for supplying a voltage required to cause the electroluminescent lamp to emit a light, and therefore, the front side and rear side lead conductors would have to be formed of metal and to be soldered to the printed wirings, as in to the conventional electroluminescent lamps, in order to ensure a required low connection resistance and a required reliability. However, as explained hereinbefore, the third electrode is provided for prevention of the mechanical vibration or noise and for the electrostatic shield, and therefore, it is sufficient if the third lead conductor makes it possible to maintain the third electrode at the same potential as that of the front side or transparent electrode. In other words, the third lead conductor is not necessarily formed of a material such as a metal which has a low electric resistance and which enables soldering. Therefore, if it is considered to constitute the third lead conductor with an extension of the third electrode, the third electrode must be formed of a relatively expensive material such as metal.